Spark plug with visual indicating
discharge bulb



United States Patent Ofilice 3,242,366 Patented Mar. 22, 1966 3,242,366SPARK PLUG WITH VISUAL INDICATING DISCHARGE BULB Alfred Candelise andJohn A. Whaley, Flint, Mich., as-

signors to General Motors Corporation, Detroit, Mich.,

a corporation of Delaware Filed Aug. 15, 1962, Ser. No. 217,086 3Claims. (Cl. 3131) This invention relates to spark plugs for internalcombustion engines, and more particularly to a spark plug adapted to bea visual indicator for its own sparking operation.

A function of the device is to eliminate the customary spark test whichis made by disconnecting the high tension cable from the plug andcausing a spark between the cable and cylinder block by cranking themotor.

In the past, indicators have been designed for the purpose of detectingspark plug operation and have taken the form of pencil-like unitscontaining a neon bulb which may be viewed through a window when thedevice is electrically connected with the spark plug electrodes. Such adevice is disclosed in US. Patent 1,957,802. In the case of thepencil-like tester, the inconvenience of carrying about an auxiliarytest device out weighs the possible advantages it may have. Furthermore,the disadvantage of having to locate the viewing window in the line ofsight of the observer has defeated attempts at its wide use. For thesereasons, a new approach was needed.

It is known that high production spark plugs have relatively thick,opaque insulators. These insulators are designed primarily for theirinsulative properties and may vary in chemical composition and structuredepending on the use of the spark plug. It is now proposed to provide aspark plug the insulator of which being manufactured so that portionsthereof become translucent whereby the spark plug serves as its ownvisual indicator of spark delivery and without adversely affecting thenormal function of such insulators.

In accordance with the invention, a spark plug having a ceramicinsulator provided with translucent wall portions and comprising atleast 85 percent alumina is adapted to receive a light source totallyconcealed therewithin. The light source is electrically energizable andadapted to be connected in series with the spark plug electrodes andwhen a sparking voltage is applied thereto, the light source isenergized illuminating the translucent wall portions producing a glow toindicate to an observer the presence of high voltage at the plug.

It is a purpose of the present invention to overcome the disadvantagesof the pencil-like tester by the use of the novel insulator whichenables an observer to detect the presence of energy at the plugregardless of the position of the insulator with respect to theobservers line of sight and regardless of Whether the device is usedduring daylight or darkness.

These and other objects will appear by reference to the followingdescription and drawing.

Referring to the drawing in detail, the spark plug is represented by alongitudinal sectional view and is equipped with a conventional metalshell 12 having threads 14 for threadably mounting the spark plug in anengine. Positioned within the metal shell 12 and secured in theconventional manner is an insulator 16. The insulator is of a ceramicmaterial composed of at least 85 percent alumina and may be glazed orunglazed depending upon the Wishes of the user. The insulator 16 isformed with a center bore using a lower portion 18 of relatively smalldiameter, a center portion 20 of larger diameter, and a top portion 22of still larger diameter which serves as a well for receipt of asuitable light source 24 herein illustrated for convenience as a neonlamp having electrode leads 26 projecting into the neon atmosphere andextending from opposite ends and being sealed by the glass bulb 28; theleads being bent to lay against the surface of the glass bulb 28 toestablish electrical contact as hereinafter described. In the lowerportion 18 of the insulator is positioned a metallic electrode 30 havinga serrated tip. The electrode center element 32 is sealed in the centerportion 20 of the insulator and connects with electrode 30 through aconductive glass seal 34. Coned coil springs 36 receive the tapered endsof the bulb 28 and electrically contact the leads 26.

The bulb 28 is biased by the springs 36 axially in a floating positionapproximately midway within the upper portion 22 of the insulator body.The springs 36 bear against the terminal 38 in one case and against thetop of the electrode center element 32 in the other, thus establishingan electrical path from the terminal 38 to the serrated tip of thecenter electrode 30. The ground electrode 40 is attached to the metalbase 12 and spaced from the serrated tip of the center electrode 30 toestablish a firing gap in the well-known manner. The springs 36 alsohave an important safety function. Should the bulb 28 accidently bebroken, the operation of the plug will not be affected. That is, theplug will continue to operate in the manner of a conventional spark gapplug in that the spark will jump across the gap at the springs. Thiswill not occur when the bulb 28 is in position because the electricalpath established by the neon lamp electrode leads 26 is of lesserelectrical resistance than that of the gap between springs 36.

It is to be understood that the insulator 16 is of a specialconstruction as will hereinafter appear. It is known that spark pluginsulators are formed of a relatively high alumina ceramic materialproviding electrical insulation between the center wire electrode andthe metal base of the spark plug. Such insulators must function in allthe ways conducive of efiicient spark plug operation. Theserequirements, it is assumed, are well-known to those skilled in the art.The present invention is concerned with providing an insulator meetingthe above requirements but in addition, providing one having theproperty of translucence, a property, to our knowledge, heretofore neverdirectly associated with spark plug insulators. This physical propertyhas been taken advantage of in the present invention.

Accordingly, we have found that it the chemical composition of theceramic material that goes into the insulator has an alumina content inexcess of percent and if insulator wall portions are molded with zonesof relatively thin cross section as shown by the circular rings 42formed in the walls of insulator 16; then a light source of normalintensity situated within such an insulator will send light through thewalls becoming visible uniformly to an observer from without. Forexample, an observer looking at the insulator 16 during the operation ofthe plug would see a high-frequency flutter or glow at the circulardepressions or rings 42. Such a glow would naturally be more apparentduring the hours of darkness. However, the preferred embodiment of theinvention as illustrated compensates for the loss in contrast during thehours of daylight by providing for shadow casting projections orshoulders 44 which increase the illumination effect at the glow rings42.

In most cases, the alumina content should be as high as possible and atleast in excess of 85 percent alumina. The wall thickness at the rings42 should, of course, be as thin as possible and still satisfy theengineering requirements of a spark plug insulator. That is, dependingupon the particular engine and the type of plug, the insulator thicknessin these regions will be designed to provide for maximum translucencyunder the circumstances. For satisfactory viewing, the current flowingthrough the neon lamp is in the range of 2 to 15 milliamperes and theinsulator thickness is in the range of .03 to .06 inch.

Although the invention has been described with reference to theparticular embodiments shown, it is not to be so limited since changesand alterations therein may be made which are within the full andintended scope of the claims which follow.

We claim:

1. Aspark plug comprising a metal base;

an insulator body adapted to receive a light source mounted on said baseand formed of a ceramic material comprising at least 85% alumina, saidinsulator body having wall portions sufliciently thin to becometranslucent, said insulator having a stepped centerbore;

a first electrode connected to said base;

a second electrode insulated from said base by said insulator and spacedfrom said first electrode;

a terminal mounted on the top of said insulator and having the lowerportion thereof extending into the insulator centerbore;

a glow discharge bulb of the rare gas type positioned in saidoenterbore, said bulb normally providing an electrical series pathbetween said second electrode and said terminal whereby a voltagesupplied across said first electrode and said second electrode that issufiicient to produce an electrical spark therebetween willsimultaneously energize said glow discharge bulb sending visible lightthrough said translucent wall portions to indicate spark plug operation;and

electrode means associated with said bulb operative to establish anauxiliary spark gap in said insulator when said bulb is inoperativethereby enabling said spark plug to continue to operate.

2. A spark plug comprising a metal base;

an insulator body adapted to receive a light source mounted on said baseand formed of a ceramic material comprising at least 85% alumina, saidinsulator body having wall portions sufficiently thin to becometranslucent, said insulator body having a stepped centerbore;

a first electrode connected to said base;

a second electrode insulated from said base by said insulator and spacedfrom said first electrode;

a terminal mounted on the top of said insulator and having the lowerportionthereof extending into the insulator centerbore;

a glow discharge bulb of the rare gas type positioned in said.centerbore and having an upper electrode lead sealed thereto projectingfrom the upper end of said tube and a lower electrode lead sealedthereto projecting from the lower end of said tube;

said upper lead and said lower lead bent to lay against the surface ofsaid bulb so that the ends of said leads face each other;

an upper coil spring positioned about the upper end of said bulb and inelectrical contact with said upper electrode lead, said upper springbeing in electrical contact with the said terminal;

a lower coil spring positioned about the lower end of said bulb and inelectrical contact with said lower lead, said lower coil spring being inelectrical contact with said second electrode;

' said bulb normal-1y providing an electrical path between said secondelectrode and said terminal where-- by a voltage supplied across saidfirst electrode and said second electrode that is sufficient to producean electrical spark therebetween will simultaneously energize said glowdischarge bulb sending visible light through said translucent wallportions to indicate spark plug operation; and

means including said springs being operative to establish an auxiliaryspark gap when said bulb is inoperative thereby enabling said spark plugto continue to operate.

3. A spark plug comprising a metal base;

an insulator body adapted to receive a light source mounted on said baseand formed of a ceramic material comprising at least alumina, saidinsulator body having wall portions sufiiciently thin to becometranslucent, said insulator body having a stepped centerbore;

a first electrode connected to said base;

a second electrode insulated from said base by said insulator and spacedfrom said first electrode;

a terminal mounted on the top of said insulator and having the lowerportion thereof extending into the insulator centerbore;

a glow discharge bulb of the rare gas type positioned in said centerboreand having an upper electrode lead sealed thereto projecting from theupper end of said tube and a lower electrode lead sealed theretoprojecting from the lower end of said tube;

said upper lead and said lower lead bent to lay against the surface ofsaid bulb so that the ends of said leads face each other;

an upper coil spring positioned about'the upper end of said bulb and inelectrical contact with said upper electrode lead, said upper springbeing in electrical contact with the said terminal;

a lower coil spring positioned about the lower end of said bulb and inelectrical contact with said lower lead, said lower coil spring being inelectrical contact with said second electrode;

said bulb normally providing an electrical series path between saidsecond electrode and said terminal whereby a voltage supplied acrosssaid first electrode and said second electrode that is sufiicient toproduce an electrical spark therebetween will simultaneously energizesaid glow discharge bulb sending visible light through said translucentwall portions to indicate spark plug operation; and

said springs cooperating to establish an auxiliary spark gap betweensaid upper lead and said lower lead when said bulb is inoperativethereby enabling said spark plug to continue to operate.

References Cited by the Examiner UNITED STATES PATENTS 1,233,630 10/1914White 313-129 XR 1,690,702 11/1928 Rotella 313129 XR 2,094,447 9/1937Doak 313124 XR 2,170,600 8/1939 V011 313201 XR 2,924,642 2/1960 Dart313144 XR 2,971,110 2/1961 Schmidt 313-221 FOREIGN PATENTS 594,506 6/1925 France.

721,722 12/1931 France.

201,225 7/ 1923 Great Britain.

203,403 9/1923 Great Britain.

293,940 3/1932 Italy.

GEORGE N. WESTBY, Primary Examiner.

DAVID J. GALVIN, Examiner.

1. A SPRAK PLUG COMPRISING A METAL BASE; AN INSULATOR BODY ADAPTED TORECEIVE A LIGHT SOURCE MOUNTED ON SAID BASE AND FORMED OF A CERAMICMATERIAL COMPRISING AT LEAST 85% ALUMINA, SAID INSULATOR BODY HAVINGWALL PORTIONS SUFFICIENTLY THIN TO BECOME TRANSLUCENT, SAID INSULATORHAVING A STEPPED CENTERBORE; A FIRST ELECTRODE CONNECTED TO SAID BASE; ASECOND ELECTRODE INSULATED FROM SAID BASE BY SAID INSULATOR AND SPACEDFROM SAID FIRST ELECTRODE; A TERMINAL MOUNTED ON THE TOP OF SAIDINSULATOR AND HAVING THE LOWER PORTION THEREOF EXTENDING INTO THEINSULATOR CENTERBORE; A GLOW DISCHARGE BULB OF THE RARE GAS TYPEPOSITIONED IN SAID CENTERBORE, SAID BULB NORMALLY PROVIDING ANELECTRICAL SERIES PATH BETWEEN SAID SECOND ELECTRODE AND SAID TERMINALWHEREBY A VOLTAGE SUPPLIED ACROSS SAID FIRST ELECTRODE AND SAID SECONDELECTRODE THAT IS SUFFICIENT TO PRODUCE AN ELECTRICAL SPARK THEREBETWEENWILL SIMULTANEOUSLY ENERGIZE SAID GLOW DISCHARGE BULB SENDING VISIBLELIGHT THROUGH SAID TRANSLUCENT WALL PORTIONS TO INDICATE SPARK PLUGOPERATION; AND ELECTRODE MEANS ASSOCIATED WITH SAID BULB OPERATIVE TOESTABLISH AN AUXILIARY SPARK GAP IN SAID INSULATOR WHEN SAID BULB ISINOPERATIVE THEREBY ENABLING SAID SPARK PLUG TO CONTINUE TO OPERATE.